President Trump is expected to nominate Kirstjen Nielsen, a cybersecurity expert and deputy White House chief of staff, to be Homeland Security secretary, a job left vacant when John F. Kelly left the department to become White House chief of staff in July, four administration officials confirmed Wednesday.

The official announcement could come as soon as Wednesday, although the officials cautioned nothing is set until Trump publicly announces her as the pick.

Nielsen, a longtime Homeland Security Department official who served as Kelly’s chief of staff when he was DHS secretary, accompanied him to the White House as his deputy.

Other contenders for the Cabinet post included Tom Bossert, Trump’s homeland security adviser, and Kevin McAleenan, the acting commissioner of U.S. Customs and Border Protection.

But Nielsen had one crucial advantage — the absolute trust and support of Kelly, to whom she grew close after volunteering to help “sherpa” him through the confirmation process earlier this year.

At the White House, however, as Kelly’s enforcer, Nielsen quickly emerged as a controversial presence.

Her detractors viewed her no-nonsense style as brusque, and complained that she could be unresponsive as she worked with Kelly to streamline West Wing operations and instill discipline in a White House often lacking structure. But her allies and fans said she was simply helping to professionalize the West Wing — the sort of necessary but thankless task that often leaves some staff members griping.

The news of Nielsen’s expected nomination was first reported by Politico.